Improvement of the performance of a power device that uses a silicon (Si) substrate (hereinafter, referred to as “silicon power device”) has conventionally been advanced aiming at controlling high frequency and high electric power. However, application of a new semiconductor material is under consideration to meet the demand for a power device with a higher performance consequent to silicon power devices being unusable under a high temperature condition, etc.
Silicon carbide (SiC) has a wide forbidden band width that is about three times as wide as that of silicon and therefore, is excellent in controllability of electrical conductivity at a high temperature; has a breakdown voltage that is higher than that of silicon by about 10-fold; and consequently, is applicable as a substrate material for a high voltage device. Furthermore, silicon carbide has an electron saturation drift velocity that is about two times as high as that of silicon and therefore, is further applicable to a device to control high frequency and high power.
According to a known method concerning a technique of forming a back face electrode of a power device using a silicon carbide substrate, silicon in the silicon carbide substrate and nickel (Ni) in a nickel film are caused to react with each other to form a reaction layer including nickel silicide; and thereby, an ohmic property is acquired between the silicon carbide substrate and the nickel film. With the ohmic electrode formed according to this method, however, free carbon (C) isolated on the surface of the ohmic electrode degrades the adhesiveness between the ohmic electrode and a metal wiring layer formed on the ohmic electrode. Consequently, a problem arises in that the metal wiring layer tends to peel off. To solve this problem, an approach below has been proposed.
For example, Patent Document 1 discloses a method according to which: on the surface of the silicon carbide substrate, a second metal film including any one of titanium (Ti), tantalum (Ta), and tungsten (W) is formed on a first metal film including nickel or a nickel alloy; and heat treatment is executed therefor. Patent Document 1 describes that: according to this method, free carbon generated by the production of a nickel silicide reacts with the second metal film to produce a carbide; therefore, the carbon component can be prevented from being isolated onto the surface of the metal film; and the ohmic electrode and the metal wiring layer can be prevented from peeling off from each other.
Patent Document 1: Japanese Laid-Open Patent publication No. 2006-344688